An analysis method using a mass spectrometry unit having a tandem mass spectrometry function can be divided broadly into two methods.
One method is a method for measuring quantities of all ionized materials for each mass-to-charge ratio (m/z) of the materials. This method is referred to as an MS (or MS1) analysis.
Another method is a method of selecting only ions each having a specific mass-to-charge ratio m/z (referred to as “precursor ion”) from among all ionized materials to separate the precursor ions from among the other ions (this process is referred to as “isolation”), further dissociating the precursor ions to produce ions (referred to as “product ion”), and then measuring the quantity of the product ions for each m/z. This method is referred to as an MSn analysis. The MSn analysis is referred to as an MS2 analysis (one time of dissociation), an MS3 analysis (two times of dissociation), . . . , and an MSn analysis ((n−1) times of dissociation) according to the number of times of repetition of the selection, the isolation, and the dissociation of the precursor ion.
In general, depending on an object to be measured, the ions of different materials can appear at the same m/z. For this reason, such materials cannot be differentiated from each other only by the MS analysis. In contrast to this, the MSn analysis reveals at what level of m/z the product ion appears, so that the MSn analysis can differentiate the materials of the objects to be measured from each other in more detail. In this way, the amount of information obtained by the MSn analysis is generally larger than the amount of information obtained by the MS analysis.
Here, the amount of information means the amount of information by which the presence or absence of a material of an object to be measured can be detected, the amount of information by which the kind of the material can be identified, or the amount of information by which the material can be quantified for each kind. In the case were an object to be measured contains a large amount of impurities or in the case where the amount of a material to be measured is very little, the amount of information obtained by the MSn analysis is larger than the amount of information obtained by the MS analysis.
However, a time required for the MSn analysis is longer than a time required for the MS analysis by the processes of isolation and dissociation. In addition, the amount of consumption of a target sample increases according to a time required for the mass spectrometry. For this reason, when a time required for the measurement is long, a material of an object to be measured is likely to be consumed during the measurement and hence information is likely to be not obtained thereafter. Thus, the measurement of the target sample needs to be efficiently performed within a limited time. In particular, an object material having a high ionization efficiency is consumed quickly. Therefore, the time required for the measurement of such an object material needs to be a short time. In this way, a trade-off relationship is recognized between the amount of information to be obtained and the time required for the measurement.
Hence, in the actual measurement, the MS analysis is combined with the MSn analysis to balance the amount of information and the time required for the measurement. For example, in Japanese Unexamined Patent Publication No. 2008-170260 is described a tandem mass spectrometry system in which, of a mass spectrum obtained by the MS analysis, only ions having a mass-to-charge ratio m/z whose peak intensity is equal to or larger than a given threshold is selected as precursor ions for the MS2 analysis to limit the number of times of performance of the MS2 analysis.
In this system, a series of procedures of performing the MS analysis once and then performing the MS2 analysis by the times of the number of the selected precursor ions are repeatedly performed. A database is searched for a mass spectrum obtained by the MS2 analysis for the m/z of the precursor ion of each time to thereby identify the kind of the material of each ion. In this way, the system described in Japanese Unexamined Patent Publication No. 2008-170260 restricts the number of times of performance of the MS2 analysis to thereby shorten the time required for the measurement. In this regard, when the precursor ion is selected in each time of the MS2 analysis, only m/z that is not identified by the last repetition of the series of procedures is selected (that is, the MS2 analysis is not again performed for the m/z once identified) to thereby further decrease the time required for the measurement. The system described in Japanese Unexamined Patent Publication No. 2008-170260 realizes an efficient analysis of a target sample by the method described above.